Barium iodide and strontium iodide crystals and scintillators implementing the same

ABSTRACT

In one embodiment, a crystal includes at least one metal halide; and an activator dopant comprising ytterbium. In another general embodiment, a scintillator optic includes: at least one metal halide doped with a plurality of activators, the plurality of activators comprising: a first activator comprising europium, and a second activator comprising ytterbium. In yet another general embodiment, a method for manufacturing a crystal suitable for use in a scintillator includes mixing one or more salts with a source of at least one dopant activator comprising ytterbium; heating the mixture above a melting point of the salt(s); and cooling the heated mixture to a temperature below the melting point of the salts. Additional materials, systems, and methods are presented.

RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.14/047,893, filed Oct. 7, 2013, which is a continuation of U.S.application Ser. No. 12/255,375, filed Oct. 21, 2008, each of which isherein incorporated by reference.

This application claims priority to provisional U.S. application serialnumber 60/988,475 filed on Nov. 16, 2007, which is herein incorporatedby reference.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to scintillator crystals, and moreparticularly to ionic iodide-containing crystals and scintillatordetectors employing the same.

BACKGROUND

Detection and classification of gamma ray emitters has attainedheightened importance in the protection of vulnerable targets andpopulaces from high energy explosives. Many nuclear explosives emitgamma rays, due to radioactive decay of the materials comprising theexplosives. However, many less harmful and non-explosive materials alsoemit gamma rays. Therefore, it is desirable to be able to identify, andwhenever possible, distinguish between the types of gamma ray emittersin an unknown material, possibly further concealed inside of a containeror vehicle of some type, such as a car, van, cargo container, etc.

Many types of materials emit gamma rays that appear very close togetheron a gamma spectrograph. Scintillator detectors use crystals that emitlight when gamma rays interact with the atoms in the crystals. Theintensity of the light emitted can be used to determine the type ofmaterial that is emitting the gamma rays. Scintillator detectors mayalso be used to detect other types of radiation, such as alpha, beta,and x-rays. High energy resolution scintillator detectors are useful forresolving closely spaced gamma ray lines in order to distinguish betweengamma emitters producing closely spaced gamma ray lines.

With respect to ytterbium as an activator dopant in scintillationradiation detectors, while some research has indicated the capacity forytterbium (II) to luminesce in response to excitation by ultravioletradiation, these findings neither demonstrate nor suggest that thecorresponding material would exhibit similar, or indeed any,radioluminescence in response to excitation by other radiation sources(particularly gamma rays and/or neutron radiation). Accordingly, to datethere is neither investigative nor demonstrative research exalting theutility (or lack thereof) of ytterbium as an activator dopant in ascintillation radiation detector capable of detecting ionizing radiationincluding gamma radiation and/or neutron radiation. Moreover, existingstudies of ytterbium (II) luminescence have often demonstrated acharacteristically “anomalous” emission band that is considereddisadvantageous to scintillation radiation detector materials configuredto detect ionizing radiation.

Detection sensitivity for weak gamma ray sources and rapid unambiguousisotope identification is principally dependent on energy resolution,and is also enhanced by a high effective atomic number of the detectormaterial. Generally, gamma ray detectors are characterized by theirenergy resolution. Resolution can be stated in absolute or relativeterms. For consistency, all resolution terms are stated in relativeterms herein. A common way of expressing detector resolution is withFull Width at Half Maximum (FWHM). This equates to the width of thegamma ray peak on a spectral graph at half of the highest point on thepeak distribution.

The relative resolution of a detector may be calculated by taking theabsolute resolution, usually reported in keV, dividing by the actualenergy of the gamma ray also in keV, and multiplying by 100%. Thisresults in a resolution reported in percentage at a specific gamma rayenergy. The inorganic scintillator currently providing the highestenergy resolution is LaBr₃(Ce), with about 2.6% at 662 keV, but it ishighly hygroscopic, its growth is quite difficult and it possessesnatural radioactivity due to the presence of primordial ¹³⁸La thatproduces betas and gamma rays resulting in interference in the gamma rayspectra acquired with LaBr₃(Ce). Therefore, it is desirable to have ascintillator detector that is capable of distinguishing between weakgamma ray sources that is more easily grown while still providing highenergy resolution.

SUMMARY

In one embodiment, a crystal includes at least one metal halide; and anactivator dopant comprising ytterbium.

In another embodiment, a scintillator optic includes: at least one metalhalide doped with a plurality of activators, the plurality of activatorscomprising: a first activator comprising europium, and a secondactivator comprising ytterbium.

In yet another embodiment, a method for manufacturing a crystal suitablefor use in a scintillator includes mixing one or more salts with asource of at least one dopant activator comprising ytterbium; heatingthe mixture above a melting point of the salt(s); and cooling the heatedmixture to a temperature below the melting point of the salts.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates two plots of relative intensity versus wavelength forseveral scintillator samples.

FIG. 2 is a plot of relative intensity versus wavelength for fivescintillator samples.

FIG. 3 is a plot of counts versus wavelength for undoped SrI₂ in pureand impure forms.

FIG. 4A is a plot of gamma ray spectra acquired with LaBr₃(Ce), SrI₂(5%Eu), and NiI(T1) scintillators of the ¹³³Ba source.

FIG. 4B is a plot of gamma ray spectra acquired with LaBr₃(Ce), SrI₂(5%Eu), and NiI(T1) scintillators of the ¹³⁷Cs source.

FIG. 5 is a chart comparing eight different measured or calculatedcharacteristics for seven different scintillator materials.

FIG. 6 is a flow chart of a method according to one embodiment.

FIG. 7 illustrates three plots of relative intensity versus time forseveral scintillator samples.

FIG. 8 is a chart of energy resolution as a function of gamma ray energyacquired for SrI₂(Eu) and LaBr₃(Ce) crystals.

FIGS. 9A-9C depict emission spectra for ytterbium-containing andeuropium-containing scintillator optic compositions, according tovarious embodiments.

FIG. 10A is a pulse height spectrum for a scintillator optic comprisingytterbium doped strontium iodide, according to one embodiment.

FIG. 10B is a scintillation decay timing plot for one embodiment of ascintillator optic comprising ytterbium doped strontium iodide.

FIG. 11A is a pulse height spectrum for a scintillator optic comprisinga mixture of ytterbium doped cesium iodide and barium iodide, accordingto one embodiment.

FIG. 11B is a scintillation decay timing plot for one embodiment of ascintillator optic comprising a mixture of ytterbium doped cesium iodideand barium iodide.

FIG. 12 depicts a flowchart of a method, according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

Several crystalline iodides are known to function usefully asscintillators, including NaI(T1), CsI(Na), CsI(T1), and LuI₃(Ce).NaI(T1) is by far the most common scintillator, being grown in largesizes by numerous companies and deployed in many commercial instruments.However, NaI(T1) offers a modest light yield, which limits the gamma rayenergy resolution which is possible (about 40,000 photons/MeV). CaI₂(Eu)and LuI₃(Ce) both have higher light yields (about 70,000-100,000photons/MeV) but are exceedingly difficult to grow and the lattercontains Lu as a constituent (which is a natural beta-emitter leading toan undesirable background count rate). Interestingly, LaI₃(Ce) has alsobeen tested as a potential scintillator, but was found to benon-emissive at room temperature.

Two materials have been found to have great utility as high energyscintillators: SrI₂(Eu) and BaI₂(Eu), which may resolve the problemsfacing the related compounds LuI₃(Ce) and CaI₂(Eu). Other relatedcompounds currently being used as scintillators include Ce-doped LaCl₃and LaBr₃, which both have very high light yields but have a loweratomic number (Z) than the iodides (Z of Cl, Br, I are 17, 35, 53,respectively)—the Z is a critical parameter since gamma photoelectricabsorption goes approximately as its fourth power, Z⁴. As a consequence,iodides are preferred as constituents in scintillators versus bromidesand chlorides, assuming other features are comparable.

In one embodiment, a scintillator detector makes use of SrI₂ or BaI₂crystals for the purpose of gamma ray detection, based on measuring theamount of scintillation luminescence generated by the material. For thispurpose, the crystal may be doped or undoped, giving rise to excitonic(undoped), perturbed excitonic (e.g., Na, Mg, Ca, Sc or other doping ofelectronically inactive species), or activator luminescence (e.g., Eu²⁺,Ce³⁺, Pb²⁺, TI⁺, Pr³⁺).

Undoped SrI₂ has a useful light yield, but its energy resolution withstandard photomultiplier tubes is only fair, due to its emission beinglong wavelength. When a Eu²⁺ activator is used, emissions in the blueregion are observed. Contrary to conventional wisdom and earlierfindings, a scintillator optic comprising SrI₂ doped with Eu, especiallyEu²⁺, has been found to provide a high energy resolution. For example, aSrI₂(Eu) crystal grown by the inventors evidenced an energy resolutionof <2.7% at 662keV, challenging the performance of LaBr₃(Ce) obtainedunder the same conditions.

An intriguing factor appears relevant to the excellent performance ofSrI₂(Eu). That is, the lattice constants for SrI₂ and EuI₂ are nearlyidentical, thus permitting high doping of Eu in SrI₂. Other favorableaspects of SrI₂ include its low melting point, 538° C., and itsorthorhombic crystal structure, which will likely be readily grown tolarge sizes.

The alpha particle-induced luminescence of BaI₂(Eu) is similar to thatof Lu₃Al₅O₁₂(Ce), but shifted to shorter wavelength.

A marked advantage of using SrI₂ or BaI₂ crystals doped with Eu is therelative ease in which the crystals can be grown in large sizes. Anotheradvantage is that SrI₂(Eu) and BaI₂(Eu) are less hygroscopic thanCaI₂(Eu) which is an important practical edge in using Sr or Ba insteadof Ca.

In a first general embodiment, a material comprises a crystal, which iscomprised of strontium iodide (doped or undoped) providing at least50,000 photons per MeV. In one particularly preferred embodiment, theenergy resolution of the crystal may be less than about 5.0% at 662 keV,as being enhanced by doping, e.g., with Ce or Eu.

In the first general embodiment, the crystal may be doped with europiumin different percentages, such as containing more than 1.6% europium,containing between about 0.5% and about 8.0% europium, and containingmore than 2.0% europium.

In addition, the europium in the crystal may be primarily Eu²⁺. The useof Eu²⁺ surprisingly provides excellent energy resolution, e.g., lessthan about 2.7%) at 662 keV. As noted above, conventional wisdom and aprevious report indicated that such energy resolution was impossible forsuch a material. To exemplify, FIG, 8 is a chart 800 depicting energyresolution as a function of gamma ray energy acquired for SrI₂(5%Eu) andLaBr₃(Ce) crystals. These crystals are the same as used below in Example4. As shown, the energy resolution of SrI₂(5%Eu) is comparable orslightly better than that of LaBr₃(Ce).

Another variation of the first general embodiment is where the crystalhas at least one dopant, selected from: cerium, praeseodymium, thallium,or lead.

The first general embodiment may further include barium in the crystal,or the crystal may provide at least 60,000 photons per MeV. Further, theresolution of the crystal may be less than about 5%, less than about4.0%, etc, at 662 keV.

In a second general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprised of strontium iodide (doped orundoped) providing at least 50,000 photons per MeV. In one particularlypreferred embodiment, the energy resolution of the crystal may be lessthan about 5.0% at 662 keV.

Also, in the second general embodiment, the scintillator optic maycontain more than 1.6% europium, may contain between about 0.5% andabout 8.0% europium, or may contain more than 2.0% europium. Inaddition, the europium may be primarily Eu²⁺. Further, the scintillatoroptic may include barium and/or calcium.

With continued reference to the second general embodiment, thescintillator optic may provide at least 60,000 photons per MeV, and mayhave a resolution of less than or about 4.0% at 662 keV.

In a third general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprised of SrI₂ and BaI₂, wherein aratio of SrI₂ to BaI₂is in a range of between 0:1 and 1:0.

In the third general embodiment, the scintillator optic may provide atleast 50,000 photons per MeV and energy resolution of less than about5.0% at 662 keV.

Further, the scintillator optic may contain europium, and the europiummay be primarily Eu²⁺. In addition, the scintillator optic may provideat least 80,000 photons per MeV, and may contain at least one dopant,selected from: cerium, praeseodymium, thallium, or lead.

In a fourth general embodiment, a scintillator radiation detectorcomprises a scintillator optic comprising barium iodide.

In the fourth general embodiment, the scintillator optic may be dopedwith at least one of cerium, praeseodymium, thallium, lead, indium, or atransition metal ion. Also, the scintillator optic may be doped with anactivator that luminesces in response to gamma radiation.

In the fourth general embodiment, the activator may include an ion whichluminesces via a 5d→4f transition or the activator may include an s² ionor a closed shell ion. Further, the activator may be a transition metalion.

In a fifth general embodiment, an iodide crystal comprises a singlemetal ion (M, M′ or M″) with the formula MI₂, M′I₃, or M″I₄, where M orM′ has an atomic number >40, but is not Y, Sc, La, Lu, Gd, Ca, Sr or Ba.M″ may or may not have an atomic number greater than 40.

In a sixth general embodiment, a scintillator optic comprises at leastone metal halide doped with ytterbium. Preferably, but not necessarily,the material is in the form of a single crystal.

The material of the sixth general embodiment preferably includes one ormore alkali halide and/or one or more alkaline earth halides as themetal halide component. Suitable alkali metals include lithium, sodium,potassium, and cesium, with cesium being particularly preferred.Suitable alkaline earth metals include beryllium, magnesium, calcium,strontium, and barium, with calcium, strontium and barium beingparticularly preferred. Suitable halides include fluorine, bromine,chlorine and iodine, with iodine being particularly preferred.

For example, in one approach at least one alkaline earth halide dopedwith an activator compound is included in the scintillator optic. Forexample, the optic may include strontium iodide doped with ytterbium asthe activator compound, and preferably doped with divalent ytterbium(a.k.a, “ytterbium (II)” or “Yb²⁺”).

In at least some approaches, the material comprises one or more alkalineearth halide salts, preferably alkaline earth iodides such as strontium(II) iodide and/or barium (II) iodide, although other alkaline earthhalides are also suitable for use in scintillator optics and should beunderstood to generally fall within the scope of the presently describedsixth general embodiment.

The alkaline earth halide(s) of the sixth general embodiment are dopedwith an activator comprising a rare-earth element. More preferably, therare-earth element is a lanthanide series element, and most preferablythe rare-earth material is ytterbium,

In more variations on the sixth general embodiment, the dopant activatormay include one or more rare earth elements, such as preferablyincluding both europium and ytterbium, In particularly preferredapproaches, the ytterbium may be included in an amount greater than theeuropium.

Without wishing to be bound to any particular theory, the inventorstheorize that advantageous energy transfer from the ytterbium dopant tothe europium dopant advantageously mitigates light-trapping otherwiseexperienced by the europium dopant in compositions excluding ytterbium,or in compositions in which the ytterbium component is present in anamount less than or equal to the europium dopant, thereby increasing theoverall light yield and suitability of the optic for application as ascintillator detector.

In the sixth general embodiment, the ground state of the activatorpreferably is characterized by a 4f¹⁴ electronic structure, a completelyfilled shell, (indicating that the ground state is a singlet, ¹S₀ state.The first excited state is therefore preferably derived from a 4f¹³5datomic state, and the absorption results from the 4f-to-5d transition.Similarly, in some embodiments but not all, the emission results from aninverse transition, i.e. 5d-to-4f.

In a seventh general embodiment, a crystal includes at least one metalhalide; and an activator dopant comprising ytterbium.

In an eighth general embodiment, a scintillator optic includes: at leastone metal halide doped with a plurality of activators, the plurality ofactivators comprising: a first activator comprising europium, and asecond activator comprising ytterbium.

In a ninth general embodiment, a method for manufacturing a crystalsuitable for use in a scintillator includes mixing one or more saltswith a source of at least one dopant activator comprising ytterbium;heating the mixture above a melting point of the salt(s); and coolingthe heated mixture to a temperature below the melting point of thesalts.

In some approaches, the crystal may be characterized by characteristicsof growth using a particular technique or methodology, such as growthfrom solution, growth from a melting process, etc. as would beunderstood by one having ordinary skill in the art upon reading thepresent descriptions. Preferably, these crystal growth characteristics(whether from melt, solution, or otherwise) include the scintillatoroptic comprising at least one single crystal.

With respect to crystal size, in some approaches the crystal growthcharacteristics include the scintillator optic comprising at least onesingle crystal having at least one face with an area of approximatelyone square centimeter (1 cm²), or more preferably the at least onesingle crystal having a volume of at least approximately one cubiccentimeter (1 cm³).

As understood herein, a “single crystal” is a solid materialcharacterized by arrangement of the constituent atoms in a crystallinelattice. The crystal furthermore exists as a single, continuous,uninterrupted region of the solid material, i.e. a single crystallinephase uninterrupted by grain boundaries, inclusions, impurities, etc. aswould be understood by one having ordinary skill in the art upon readingthe present descriptions.

Another way of expressing the “single crystal” concept describedpresently is to consider a “single crystal” as being embodied as asingle grain of the corresponding crystalline material and having thecorresponding spatial dimensions. In one exemplary instance within thescope of the sixth general embodiment, a “single crystal” ofytterbium-doped strontium iodide” is a single grain of solid materialembodied as a crystalline lattice throughout which the strontium, iodineand ytterbium atoms are distributed. The single grain is uninterruptedby inclusions, impurities or grain boundaries, and is furthercharacterized by having at least two, and preferably three,perpendicularly-oriented (or orthogonal, in other terms) dimensions eachbeing at least about one centimeter in length, i.e. an area of at leastabout one square centimeter or a volume of at least about one cubiccentimeter, respectively.

The single crystal may be joined to one or more additional singlecrystals in a complex, with each crystal being separated by one or morecrystal grain boundaries.

Any of the general embodiments may include further limitations asdirected below. In addition, combinations of the additional limitationsdirected below may be combined to create even more permutations andcombinations of features.

EXAMPLES

To demonstrate various embodiments of the present invention, severalexamples are provided bellow. It should be appreciated that these arepresented by way of nonlimiting example only, and should not beconstrued as limiting.

Example 1

Strontium iodide and barium iodide crystals were grown in quartzcrucibles using the Bridgman method. The melting points of SrI₂ and BaI₂are 515° C. and 711° C., respectively; both possess orthorhombicsymmetry while calcium iodide is hexagonal. All crystals described inthis section were doped with 0.5 mole % europium and were several cubiccentimeters per boule, then cut into ˜1 cm³ pieces for evaluation.Barium iodide as-supplied powder, 99.995% pure ultradry (Alfa Aesar) wasyellowish in color (thought to be due to oxide or oxyiodidecontamination). Crystals grown directly from as-supplied powdersretained a dark coloration (referred to henceforth as “first crystal”),Zone refining rendered the starting powders colorless, and the resultingpure powders were used to grow several crystals (referred to as “secondcrystal,” although several were grown following this procedure).Finally, an ultrapurification method was used to grow a BaI₂(Eu)crystal, referred to as “third crystal.”

Radioluminescence spectra were acquired using a ⁹⁰Sr/⁹⁰Y source (averagebeta energy ˜1 MeV) to provide a spectrum expected to be essentiallyequivalent to that produced by gamma excitation. Radioluminescencespectra were collected with a spectrograph coupled to athermoelectrically cooled camera and corrected for spectral sensitivity.The beta-excited luminescence of SrI₂(0.5% Eu) compared to that of astandard scintillator crystal, CsI(T1), is shown in FIG. 1 in the upperplot 102, along with a SrI₂(Eu) crystal grown at RMD. It possesses asingle band centered at 435 nm, assigned to the Eu²⁺d→f transition, andan integrated light yield of 93,000 photons/MeV. FIG. 1 in the lowerplot 104 shows beta-excited luminescence spectra of three BaI₂(Eu)crystals compared to a CsI(T1) standard crystal, The Eu⁺ luminescence at˜420 nm is enhanced in the second BaI₂(Eu) crystal, while the ˜550 nmband is reduced, and for the third crystal, the ˜550 nm band is entirelyabsent. It is notable that the overall light yield is highest for thefirst crystal; its integral light yield (including both the 420 nm andthe 550 nm bands) is 60,000 photons/MeV, The weak band at 550 nm may beassigned to an impurity-mediated recombination transition.

Example 2

Calcium iodide and strontium bromide crystals were grown via theBridgman method, with 0.5% Europium doping. The CaI₂(Eu) crystal issubstantially opaque due to optical scatter, considered unavoidable dueto its platelet crystal structure. Its radioiuminescence spectrum wasmeasured at 110,000 Ph/MeV, and is shown in the chart 200 of FIG. 2.SrBr₂(Eu) is an orthorhombic crystal with good optical properties,however, its light yield so far is low (˜25,000 Ph/MeV). Allradioiuminescence spectra reported herein were acquired with a ⁹⁰Sr/⁹⁰Ysource (˜1 MeV average beta energy) and emission spectra were collectedusing a Princeton Instruments/Acton Spec 10 spectrograph coupled to athermoelectrically cooled CCD camera.

Example 3

Undoped strontium iodide was grown and zone-refined. The luminescencespectrum, shown in FIG. 2, is unchanged between pure and impure segmentsof the boule, however, the pulse height spectrum of the purer section isslightly higher. Pulse height measurements, shown in the chart 300 ofFIG. 3, were acquired using a Hamamatsu R329EGP PMT (QE at 550 nm of15%). The signals from the PMT anode were collected on a 500 Ω resistor,shaped with a Tennelec TC 244 spectroscopy amplifier (shaping time of 8μs) and then recorded with the Amptek MCA8000-A multi-channel analyzer.The emission is likely due to self-trapped excitons, as it is presentfor all un-doped samples.

Example 4

A scintillator crystal of strontium iodide doped with 5% europium, ascintillator crystal of LaBr₃(Ce), and a scintillator crystal ofNiaI(T1) were acquired and exposed to a ¹³³Ba source. Acquisitionparameters (e.g., shaping time, gain) were optimized for each crystal togive the best results for the particular crystal. The resulting gammaray spectra are shown in the chart 400 of FIG. 4A. As shown, the energyresolution of the SrI₂(Eu) is better than the energy resolution of theLaBr₃(Ce) in the low energy region.

Example 5

The same crystals used in Example 4 were exposed to a ¹³⁷Cs source,which is primarily monoenergetic. Again, acquisition parameters (e.g.,shaping time, gain) were optimized for each crystal to give the bestresults for the particular crystal. The resulting gamma ray spectra areshown in the chart 402 of FIG. 4B. As shown, the energy resolution ofthe SrI₂(Eu) is comparable to the energy resolution of the LaBr₃(Ce).

Example 6

The same LaBr₃(Ce) and SrI₂(Eu) crystals used in Example 4 were exposedto a ¹³⁷Cs source, which is primarily monoenergetic. Again, acquisitionparameters (e.g., shaping time, gain) were optimized for each crystal togive the best results for the particular crystal. The resulting gammaray spectra are shown in FIG. 4B. As shown, the energy resolution of theSrI₂(Eu) is comparable to the energy resolution of the LaBr₃(Ce).

Example 7

Several crystals of barium iodide were grown and characterized. Theradioluminescence of BaI₂(Eu) typically shows both a long-wave band,similar to that seen in undoped SrI₂, as well as the BaI₂(Eu) band shownin FIG. 2. The long-wave band, thought to be related to self-trappedexciton luminescence, is reduced as the Eu doping level is increased.However, even for crystals exhibiting only Eu luminescence, gamma lightyields and energy resolution so far are modest (see the chart 500 ofFIG. 5).

Example 8

Barium Bromide crystals were grown doped with Eu, but the light yieldsare <30,000 Ph/MeV. While it may be possible for the performance of BaI₂and BaBr₂ to be improved, but energetic considerations, such as relativepositions of the Eu²⁺ states within the bandgap, may limit light yields.For example, the Eu²⁺ excited state in BaI₂ may be too close to theconduction band to compete effectively with residual shallow traps,while this matter is resolved in SrI₂since the Eu²⁺ excited state isslightly lower with respect to the conduction band.

Therefore, of the alkaline earth halides, SrI₂(Eu) appears mostpromising due to its very high light yield, good optical properties,ease of growth, high achievable doping with Eu²⁺, Z_(eff) higher thanLaBr₃(Ce), excellent light yield proportionality and demonstrated energyresolution of <2.7% at 662 keV. Cab has not been effectively grown inlarge sizes and SrBr₂ has a low Z_(eff), while BaI₂ and BaBr₂ have notdemonstrated adequate light yields for high energy resolution.

Example 9

Decay times were acquired using a flashlamp-pumped Nd:YAG laser usingthe 4^(th) harmonic at 266 run, and 20 ns FWHM pulses. Luminescence wascollected with a monochromator coupled to an R928 Hamamatsu PMT and readout by an oscilloscope. In SrI₂(Eu), the Eu²⁺ band decays with a 1.2microsecond time constant as shown in FIG. 7, top plot 702. FIG. 7,middle plot 704, shows that for the BaI₂(Eu) crystal grown withas-received powder, the Eu²⁺ decay is about 450 ns while theimpurity-mediated luminescence is slower, and cannot be fully integratedwithin an 8 μs shaping time. It is interesting that a component of theimpurity-mediated recombination pathway is prompt (pulsewidth-limited)proceeding directly by trapping carriers from the conduction and valenceband, but there is also a component that forms by depopulating the Eu²⁺excited state (possibly electrons trapped initially at Eu²⁺ are able tothermally de-trap to the conduction band), as revealed by a rise-timecomponent observed for 600 nm detection. Also, the impurity-mediateddecay is very slow, on the tens of microseconds timescale (perhaps dueto an exciton experiencing a triplet to singlet spin-forbiddentransition). For Sr I₂(Eu), the Eu²⁺ decay is about 770 ns, as shown inFIG. 7, bottom plot 706, effectively lengthened due to the reduction ofde-trapping and excitation transfer to the impurity-mediatedrecombination pathway.

Example 10

Referring now to FIGS. 9A-9C, which depict excitation spectra forseveral embodiments of a scintillator optic comprising halide salt(s)doped with ytterbium as an activator, it is apparent that thesecompositions exhibit advantageous excitation spectra for application inradiation detection endeavors.

For example, and according to various embodiments, the exemplaryscintillator optic includes at least one metal cation and an at leastone anion selected from a group consisting of: a borate, a carbonate, anitrate, an oxide, a halide, a phosphate, a sulfide, a selenide, and atelluride. More specifically, the halides are preferably salts selectedfrom one or more of magnesium iodide; calcium iodide strontium iodide;barium iodide; lithium iodide; sodium iodide; potassium iodide; rubidiumiodide; and cesium iodide.

Referring now to FIG. 9A, exemplary beta-excitation spectra forstrontium iodide-containing compositions are compared, highlighting theunique spectral characteristics of strontium iodide doped withapproximately 0.5% europium, 0.5%) ytterbium, and undoped, asrespectively indicated by the dotted, dashed and solid lines.

Notably, according to the experimental data summarized in FIG. 9A, thestrontium iodide doped with europium exhibits an emission intensity ofat least approximately 45,000 photons/MeV, while the strontium iodidedoped with ytterbium exhibits an intensity of at least approximately56,000 photons/MeV in response to excitation by electromagneticradiation within the depicted spectral band (e.g. about 100-700 nmwavelength).

As illustrated in FIG. 9A, while all three embodiments exhibit theband-edge emission starting at approximately 380 nm, in the undopedstrontium iodide material, this weak emission is dominated by thestronger emission in the region spanning approximately 500-650 nm(presumably due to defect-mediated exciton action).

By contrast, the excitation spectra for strontium iodide doped witheither europium or ytterbium exhibit strong emission peaks atapproximately 380 nm, with experimental results indicating increasedlight yield is at least partially due to the dopant activator shiftingthe dominant excitation events toward the corresponding emission peak ofthe dopant (approximately 380 nm for both europium and ytterbium).

With reference to FIG. 9B, exemplary beta-excitation spectra for cesiumbarium iodide doped with 1.0% ytterbium (solid line) and 0.5% ytterbium(dotted line) are illustrated for comparison. The spectra depicted inFIG. 9B reveal a strong emission band at approximately 414 nm. Theexperimental data from which FIG. 9B is compiled indicate theytterbium-doped embodiments exhibit an emission intensity ofapproximately at least about 51,000 photons/Me V at a dopingconcentration of about 0.5%, while a doping concentration of about 1.0%yields an emission intensity of approximately about 50,000 photons/MeVor more, in various approaches.

Now with reference to FIG. 9C, an exemplary comparison of ultraviolet(UV) excitation spectra for strontium iodide doped with 0.5% europium(dotted line), strontium iodide doped with 0.5% ytterbium (dashed line),and cesium barium iodide doped with 0.5% ytterbium (solid line) aredepicted, according to one embodiment. Each of the ytterbium-dopedembodiments (regardless of halide salt identity) exhibited strongexcitation peaks at approximately 414 nm, which correspond to excitationof Yb²⁺. while the europium-doped strontium iodide exhibits anexcitation peak at approximately 432 nm, corresponding to excitation ofEu²⁺, as would be expected. Notably, however, the ytterbium-dopedstrontium iodide sample also exhibits a characteristic shoulder atapproximately 432 nm, which is theorized to be due to minute presence ofeuropium impurities in the starting materials from which the crystalswere manufactured,

For example, and as shown in FIGS. 10A and 11A, exemplary pulse heightspectra are displayed. The exemplary spectra correspond to ascintillator optic comprising strontium iodide (FIG. 10A) or cesiumbarium iodide (FIG. 11A), each doped with approximately 0.5% ytterbium.For each composition, the spectrum was generated using a ¹³⁷Cs radiationsource.

Similar to the depictions in FIGS. 10A and 10B, FIGS, 11A and 11Billustrate the pulse height spectrum (again, corresponding to ¹³⁷Cs) andemission lifetime curve for a scintillator optic comprising cesiumbarium iodide doped with approximately 1.0% ytterbium, according to oneembodiment.

Referring again to FIGS. 10A and 10B, these FIGS. depict a pulse heightspectrum and scintillator decay timing, respectively, for strontiumiodide doped with Yb²⁺ (0.5%) according to one embodiment. The FIGS.demonstrate how the scintillator optic compositions disclosed hereincomprising strontium iodide doped with ytterbium exhibit pulse heightspectra and emission lifetime curves indicating suitability forutilizing these materials for the detection of ionizing radiation suchas gamma rays in various embodiments.

For example, as shown in FIG. 10A, the exemplary strontium iodidescintillator optic (a substantially pure single crystal ofytterbium-doped strontium iodide having a volume of approximately 130cubic millimeters) doped with ytterbium (II) exhibits a resolution ofapproximately 4.65% at 662 keV, in one embodiment.

Similarly, and as shown in FIG. 10B, the exemplary scintillator opticcomprising strontium iodide doped with ytterbium exhibits fast, mid-slow(a.k.a. “medium”) and slow decay components. Each decay component isrespectively characterized by a decay time of about 62 (±4) nanosecondsfor the fast component, corresponding to about 1.1% of the total lightproduced by the scintillator; about 400 (±15) nanoseconds for themid-slow component, corresponding to approximately 32.3%) of the totallight produced by the scintillator; and about 920 (±20) nanoseconds forthe slow component, corresponding to the remaining approximately 66.6%of the light produced by the scintillator.

Referring now to compositions of the scintillator optic including cesiumbarium iodide doped with ytterbium as disclosed herein, an exemplarypulse height spectrum is shown in FIG. 11A, according to one embodiment.The exemplary scintillator optic including cesium barium iodide dopedwith ytterbium is characterized by an energy resolution of approximately5.7% at about 662 keV.

As depicted in FIG. 11B, the exemplary scintillator optic comprisingcesium barium iodide doped with ytterbium exhibits fast, mid-slow(a.k.a. “medium”) and slow decay components. Each decay component isrespectively characterized by a decay time of about 74 (±2) nanosecondsfor the fast component, corresponding to about 1.4% of the total lightproduced by the scintillator; about 870 (±70) nanoseconds for themid-slow component, corresponding to approximately 30.9% of the totallight produced by the scintillator; and about 2770 (±100) nanosecondsfor the slow component, corresponding to the remaining approximately57.7% of the light produced by the scintillator.

Illustrative Method

Now referring to FIG. 6, a method 600 according to one embodiment isshown. As an option, the present method 600 may be implemented in thecontext and functionality architecture of the preceding descriptions. Ofcourse, the method 600 may be carried out in any desired environment. Itshould also be noted that the aforementioned definitions may applyduring the present description.

With continued reference to FIG. 6, in operation 602, strontiumiodide-containing crystals are mixed with a source of Eu²⁺. Any type ofstrontium iodide-containing crystals may be used, and the source of Eu²⁺may be of any type.

In operation 604, the mixture is heated above a melting point of thestrontium iodide-containing crystals. The melting point may be differentthan that of Eu²⁺ alone or It may be different than a melting point ofstrontium iodide-containing crystals alone.

In operation 606, the heated mixture is cooled near the seed crystal forgrowing a crystal. The grown crystal may contain more than 1.6%europium, more than 2.0% europium, or between about 0.5% and about 8.0%europium, Further, the europium in the grown crystal may be primarilyEu²⁺.

Now referring to FIG. 12, a method 1200 according to one embodiment isshown, As an option, the present method 1200 may be implemented in thecontext and functionality architecture of the preceding descriptions. Ofcourse, the method 1200 may be carried out in any desired environment.It should also be noted that the aforementioned definitions may applyduring the present description.

The method 1200 is described below primarily with respect to anembodiment including strontium iodide doped with ytterbium (II).However, those having ordinary skill in the art will appreciate thatsimilar techniques using similar materials that a skilled artisanreading these descriptions would appreciate as suitable for use in ascintillation radiation detector also apply to, and are within the scopeof, the present disclosures.

More particularly, it is to be appreciated that the presently disclosedtechniques are also applicable to manufacturing scintillation radiationdetector materials comprising metal halide(s) other than strontiumiodide, such as strontium bromide, calcium bromide, calcium iodide,barium iodide, cesium iodide, cesium bromide, barium chloride, etc., andmixtures thereof, as would be understood by one having ordinary skill inthe art upon reading the present descriptions.

Similarly, the presently disclosed techniques are also applicable tomanufacturing scintillation radiation detector materials comprisingsalt(s) other than the above-disclosed metal halides. For example, invarious approaches the scintillation radiation detector material mayinclude one or more salts each comprising a metal cation and amonoelemental anion, such as metal carbonates, metalborates, metalsulfides, metal selenides, metal tellurides, metal oxides, metalphosphates, metal nitrides, etc. as would be understood by one havingordinary skill in the art upon reading the present descriptions.

With continued reference to FIG. 12, in operation 1202, strontiumiodide-containing crystals are mixed with a source of Yb²⁺. Any type ofstrontium iodide-containing crystals may be used, and the source of Yb²⁺may be of any type.

In operation 1204, the mixture is heated above a melting point of thestrontium iodide-containing crystals. The melting point may be differentthan that of Yb²⁺ alone, or may be different than a melting point ofstrontium iodide-containing crystals alone, in various approaches.

In operation 1206, the heated mixture is cooled near the seed crystalfor growing a crystal. The grown crystal may contain more than 1.6%Ytterbium, more than 2.0% Ytterbium, between about 0.5% and about 10%Ytterbium, or more than 10% ytterbium, in various embodiments. Further,the Ytterbium in the grown crystal may preferably be primarily Yb²⁺.

In further embodiments, the method may additionally and/or alternativelyinclude incorporating europium and ytterbium, preferably in the form ofEu²⁺ and Yb²⁺, respectively, into the scintillation radiation detectormaterial. In preferred approaches, the inclusion of both europium andytterbium in combination advantageously reduces the frequency oflight-trapping events characteristic of scintillation radiation detectormaterials including solely europium as an activator dopant.

Without wishing to be bound to any particular theory, the inventorspropose that embodiments including a mixture of ytterbium and europiumdopants in combination mitigate the occurrence and detrimental impact oflight trapping in scintillation radiation detectors because when aphoton is first absorbed by a ytterbium ion, the excited ytterbium tendsto act on physically proximate europium ions via dipole interaction(s).These dipole interactions effectively transfer energy from the ytterbiumion to the europium ion, which subsequently emits the energy asradiation to provide the desired radiation detection capability on thescintillator material. Importantly, this method of dipole interactionenergy transfer mitigates risk of a light-trapping event occurring, asis commonly the case when a europium ion directly absorbs a photon froma radiation source (rather than absorbing the energy via dipoleinteractions as with ytterbium).

Those having ordinary skill in the art will also appreciate thatytterbium is relatively unstable as a divalent ion, as compared to thetrivalent ytterbium state (i.e. Yb³⁺, ytterbium (III)). As a result, itis a common occurrence for manufacturing methods to incorporateytterbium into the scintillator material in both these oxidation states,while it is preferred that the scintillator optics produced according tothe present disclosures exclusively incorporate divalent ytterbium inlieu of including trivalent ytterbium. Without wishing to be bound toany particular theory, the inventors note that incorporation oftrivalent ytterbium is believed to detriment performance of theresulting scintillator optic because the trivalent ytterbium ions do notexhibit the preferred scintillation behavior advantageous forapplication as a scintillation radiation detector, particularly adetector of ionizing radiation like gamma rays and/or neutron radiation.

In Use

Embodiments of the present invention may be used in a wide variety ofapplications, and potentially any application in which high light yieldor high resolution is useful

Illustrative uses of various embodiments of the present inventioninclude, but are not limited to, applications requiring radiationdetection. Search, surveillance and monitoring of radioactive materialsare a few such examples. Various embodiments can also be used in thenuclear fuel cycle, homeland security applications, nuclearnon-proliferation, medical imaging, etc.

Yet other uses include detectors for use in treaty inspections that canmonitor the location of nuclear missile warheads in a nonintrusivemanner. Further uses include implementation in detectors on buoys forcustoms agents at U.S. maritime ports, cargo interrogation systems, andinstruments that emergency response personnel can use to detect orsearch for a clandestine nuclear device.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A crystal, comprising at least one metal halide;and an activator dopant comprising ytterbium.
 2. The crystal of claim 1,wherein the metal halide comprises an alkaline earth metal.
 3. Thecrystal of claim 2, wherein the alkaline earth metal comprises anelement selected from a group consisting of strontium, barium andcalcium.
 4. The crystal of claim 2, wherein the halide comprises iodine.5. The crystal of claim 1, wherein the metal comprises an alkali metal.6. The crystal of claim 5, wherein the alkali metal comprises cesium. 7.The crystal of claim 5, wherein the halide comprises iodine.
 8. Thecrystal of claim I, wherein the dopant is present in an amount rangingfrom between about 0.5% and about 10.0%.
 9. The crystal of claim 1,wherein the ytterbium is primarily Yb²⁺.
 10. The crystal of claim 1,further comprising at least one additional metal halide.
 11. The crystalof claim 10, wherein the additional metal halide is selected from agroup consisting of a strontium halide, a barium halide and a calciumhalide.
 12. The crystal of claim I, further comprising at least oneadditional activator dopant, wherein the additional activator dopantexcludes ytterbium.
 13. The crystal of claim 12, wherein the additionalactivator dopant comprises europium,
 14. The crystal of claim 13,wherein the europium is primarily Eu²⁺.
 15. The crystal of claim 12,wherein the additional activator dopant is present in an amount lessthan an amount in which the activator dopant is present.
 16. Ascintillator optic, comprising: at least one metal halide doped with aplurality of activators, wherein the plurality of activators comprisinga first activator and a second activator, wherein the first activatorcomprises europium, and wherein the second activator comprisesytterbium.
 17. A method for manufacturing a crystal suitable for use ina scintillator, the method comprising: mixing one or more salts with asource of at least one dopant activator comprising ytterbium; heatingthe mixture above a melting point of the salts; and cooling the heatedmixture to a temperature below the melting point of the salts.
 18. Themethod of claim 17, wherein the one or more salts comprise at least onemetal cation and an at least one anion selected from a group consistingof: a borate, a carbonate, a nitrate, an oxide, a halide, a phosphate, asulfide, a selenide, and a telluride.
 19. The method of claim 17,wherein the one or more halide salts comprise at least one of: magnesiumiodide; calcium iodide; strontium iodide; barium iodide; lithium iodide;sodium iodide; potassium iodide; rubidium iodide; and cesium iodide. 20.The method of claim 17, wherein the at least one dopant activatorfurther comprises europium present in an amount less than the ytterbium.